Four wheel drive has become increasingly popular in automobiles as well as utility and recreational vehicles because of performance advantages such as increased traction and enhanced handling compared to conventional two wheel drive, particularly under adverse driving conditions.
In a front-rear-locked four wheel drive mode, with a fixed drive ratio of 1:1 between the front and rear axles, traction would be superior to that of two wheel drive because nominally the drive power would be distributed equally amongst all four wheels. However this holds true only under ideal conditions, such as straight line travel and closely matched wheel diameters: under real driving conditions this ideal is rarely realized. Front-rear-lock has proven impractical and unsatisfactory for many normal driving conditions, mainly due to potential "wind up", i.e. uncontrolled interaxle power imbalances which can waste power and cause excessive tire wear from drag (a) during cornering due to the front-rear difference in wheel travel distance and rpm, and (b) in straight line travel due to differences in effective front-rear tire diameter.
These considerations have been addressed largely through two two general types of interaxle mechanisms, known as the center differential type and the "hang-on" type.
In the center (or intermediate) differential type neither axle is positively coupled to the engine: the driving torque is split differentially between the two axles, thus allowing interaxle slippage. This arrangement suffers from the disadvantage that when torque is lost at one axle, typically when traction is lost at one or both wheels of one axle, such as on mud, gravel or ice, and wheel spinning occurs, the loss of torque is equally reflected to the other axle with the result that even if the wheels of the second axle have good traction, full engine power for pulling the vehicle out of an immobile situation cannot be directed to the second axle. This problem has been addressed by such remedial measures as temporarily locking or overriding the center differential to temporarily revert to positive coupling of one or both axles in order to restore lost traction. Examples of center differential type four wheel drive systems are disclosed in U.S. Pat. No. 4,566,544 to Suzuki, No. 4,618,022 to Hayashi and No. 4,627,513 to Tutzer.
In the conventional "hang-on" type configuration, a main drive axle is positively coupled to the motive power source in the manner of a normal two wheel drive vehicle, while an auxiliary power path to the other axle through an interaxle transfer gearbox is made selectable so that the driver may choose between a two wheel drive mode and a front-rear-locked four wheel drive mode.
Four wheel drive systems may also be classified as either (a) part time four wheel drive wherein an auxiliary drive axle may be fully disengaged to revert to two wheel drive, or (b) full time four wheel drive, also known as all wheel drive, wherein all road wheels receive drive power continuously in some proportion, not necessarily equal. Full time four wheel drive is exemplified in U.S. Patents to Mueller: No. 4,691,593 in a single "hang-on" configuration utilizing a centrifugal clutch and 4,763,747 with dual hang-on transfer units, one associated with each rear wheel.
Efforts to attain satisfactory performance in full time four wheel drive under a variety of driving conditions has required development of complex and sophisticated automatic control systems to continuously proportion drive torque optimally amongst the road wheels in response to numerous vehicle operating parameters such as steering angle, vehicle speed, axle speed differences, throttle settings, charging air pressure and predicted drive wheel friction coefficients. Continuous four wheel drive control is disclosed by Stelter in U.S. Pat. No. 4,754,853 and No. 4,792,011.
Compared to full time four wheel drive systems, part time four wheel drive can generally be implemented with less complexity. In a basic conventional form, the auxiliary driveshaft is driven via a manually engaged dog clutch, which, if not of the synchronized type, may require the vehicle to be stopped for changeover. A more sophisticated automatic auxiliary drive transfer mechanism is exemplified in U.S. Pat. No. 4,567,061 to Yamakawa et al in which changeover between two wheel drive and front-rear-locked four wheel drive is implemented by means of a disengagement clutch in the auxiliary drive train under automatic control responsive to sensed vehicle speed and steering angle. Suzuki in U.S. Pat. No. 4,560,025 discloses a part time type four wheel drive system having automatic locking clutches at each of the two front wheels.